1. Field of the Invention
This invention relates to the production of novel microcapsular coating compositions containing finely divided silica particles having a hydrophobic surface and to microcapsule coated papers made therefrom.
A preferred use of the microcapsular coating composition is in the production of pressure-sensitive carbonless copy papers having a transfer coating applied to one surface fo use in multi-part forms. The invention will be described hereinafter in relation to the production of such copy papers.
2. Prior Art
Pressure-sensitive carbonless copy paper, briefly stated, is a standard type of paper wherein during manufacture the backside of a paper substrate is coated with what is referred to as CB or transfer coating, the CB coating containing a solution of one or more color precursors, generally an oil solution, in microencapsulated form. At the same time, the front side of the paper substrate is coated during manufacture with what is referred to as a CF or receptor coating which contains one or more color developers. Both the solution of color precursor and the color developer remain in the coating compositions on the respective back and front surfaces of the paper in colorless form. This is true until the CB and CF coatings are brought into abutting relationship and sufficient pressure, as by a typewriter, is applied to rupture the CB coating to release the solution of color precursor. At this time the color precursor solution transfers to the CF coating and reacts with the color developer therein to form an image. Patents relating to conventional methods of preparing carbonless paper products and to the preparation CF and CB coating composition are:
U.S. Pat. No. 2,712,507 (1955) to Green.
U.S. Pat. No. 2,730,456 (1956) to Green et al.
U.S. Pat. No. 3,455,721 (1969) to Phillips et al.
U.S. Pat. No. 3,466,184 (1969) to Bowler et al.
U.S. Pat. No. 3,672,935 (1972) to Miller et al.
Manifold forms utilizing pressure-sensitive carbonless copy papers have been in common use commercially for several years. Such copy papers are adaptable to the production of multi-part forms and have in many instances replaced the older forms utilizing carbon paper. Printing or production of the carbonless forms by photocopying has been done prior to the application of the CB or CF coatings. However, conventionally it has been desirable to print the form over the CF coating subsequent to the application of one or both of the CB or CF coatings. The CB side is normally not printed. This is especially true when it is desirable to produce the form by photocopying after the paper has been cut into sheets.
One such photocopying operation has been done on sheet fed Xerox 9200 or Xerox 9400 copiers. Both of these copiers utilize a pressure nip between a photoreceptor belt and a transfer roll to assist transfer of the xerographic developer image from a photoreceptor belt on which the developer image was formed to a sheet of paper. If, as in conventional practice, the sheet has been previously coated with microcapsules, the coated paper may contain small amounts of unencapsulated oil as may be produced by inadvertent rupture of the microcapsules. The unencapsulated oil transfers to the transfer roll with which it is in contact. The oil accumulates toner, stilt materials, paper dust and fibers and finally this accumulation transfers to the photoreceptor belt where it causes a "speck" to appear repetitively on subsequently imaged sheets. The appearance of such specks on a form is objectionable.
In addition to the patents cited supra which disclose conventional preparation of transfer (CB) coatings and receptor (CF) coatings, a number of other patents disclose the use of absorbent pigments in combination with a releasable oil as follows:
U.S. Pat. No. 2,929,736 (1960) to Miller
U.S. Pat. No. 2,980,941 (1961) to Miller
U.S. Pat. No. 3,481,759 (1969) to Ostlie
U.S. Pat. No. 3,776,864 (1973) to Woerner
U.S. Pat. No. 4,089,547 (1978) to Brynko et al.
U.S. Pat. No. 4,154,462 (1979) to Golden et al.
U.S. Pat. No. 2,929,736 is illustrative of a number of self-contained copying sheets wherein a coating containing an oil solution of a color precursor in microcapsules and isolated therefrom a reactive pigment; i.e., clay, is applied to a paper base. U.S. Pat. No. 2,980,941 discloses a microcapsular coating containing oil in the microcapsules and Fuller's earth particles. U.S. Pat. No. 3,481,759 discloses a transfer paper wherein the microcapsules in the transfer (CB) coating contain a dye precursor and the coating contains a powdered coreactant which reacts with the dye precursor from inadvertently ruptured microcapsules to form a colorless dye. U.S. Pat. No. 3,776,864 discloses a transfer ink containing a dye and a filler to prevent the coating from having a greasy surface. U.S. Pat. No. 4,089,547 discloses manifold receptor sheets containing hydrophilic fumed silicon dioxide particles. U.S. Pat. No. 4,154,462 discloses the preparation of transfer sheets having a CB microcapsular coating. The microcapsules contain an oil solution of a dye intermediate, i.e., color precursors, and the coating contains a particulate oil-absorptive material. The oil-absorptive materials are hydrophilic pigment particles of a particle size normally 0.1 to 5 microns and include such pigments as clays, talc or silica. These pigments were added to the coating composition to permit writing on the coated substrate without interference from oil released by ruptured microcapsules.
It is important to note that none of the above cited prior art patents were concerned with the problem of preventing specking during printing of CB coated sheets, a problem which was solved by the use of microcapsular coating compositions containing finely divided hydrophobic silicas. Further to this, none of the above patents suggest the use of hydrophobic treated finely divided silicas as oil absorbers and/or detackifiers.
I have found that by including hydrophobic finely divided silica in the CB coating composition, the tendency for the formation of specks during production of forms by photocopying of papers produced from these coating compositions was substantially reduced. Runs of up to 50,000 copies have been made without the appearance of specks, whereas specking of paper containing no hydrophobic silica usually occurred before the 7,000th copy. The inclusion of finely divided hydrophobic silica by reducing specking improves the appearance of the form and reduces printing press "downtime" for cleanup of the photoreceptor belt and transfer roll.